Aging Affects Vitamin C Status
My research focus, under the direction of Dr. Tory Hagen, is the decline in vitamin C levels seen with
increasing age. Vitamin C is an essential compound for human health whose attributes were persuasively
advocated by Linus Pauling for over 25 years. With a structure similar to many carbohydrates, vitamin C
is unique in that it can undergo reversible oxidation reactions. Additionally, the chemical properties
of vitamin C allow it to react with a wide variety of potentially harmful reactive oxygen species.
Vitamin C is one of the most powerful biological antioxidants, and it is present in high concentrations
in cells and extracellular fluids. Additionally, vitamin C has biosynthetic roles in which it participates
in a number of enzyme reactions for the generation of life-essential molecules. The importance of vitamin C
is seen clearly in scurvy, a life-threatening vitamin C deficiency disease that results in loss of energy,
depression, mood disorders, poor wound healing, and connective tissue disorders, culminating in death.
In a normal, healthy individual eating a diet rich in vitamin C, we might expect adequate levels of vitamin C for normal cell functions. Any condition that would decrease these levels, such as smoking or chronic diseases, would therefore result in a decline in cell function. Of particular interest to our laboratory is the observation that vitamin C levels appear to decline in the elderly. This has been observed not only in human plasma, but also in plasma and tissues in rats, our model system. Why do vitamin C levels decline with age? The most straightforward answer is that vitamin C uptake declines with age. In other words, our bodies don't have the same capacity to absorb vitamin C, so vitamin C levels in the plasma and tissues become lower. This has been supported by a recent meta-analysis, which showed that the elderly had much lower plasma vitamin C levels than younger adults following a given dose of vitamin C. This suggests that vitamin C absorption in the gut declines with age. Vitamin C absorption is governed by the same process that allows vitamin C to enter cellsmembrane transport. The mechanism responsible for vitamin C transport in the gut and all other tissues is a Sodium-dependent Vitamin C Transporter known as SVCT. Two isoforms of this protein, SVCT1 and SVCT2, are known to exist in every mammal, including humans. Since this protein was only discovered a few years ago, we do not know much about its characteristics. In particular, nobody knows how expression or activity of this protein changes with age. It is our hypothesis, and the focus of my work in the lab, that the decline in age-related vitamin C levels is due to a loss of vitamin C transport, primarily due to changes in SVCT activity.
We conducted most of our experiments in cells isolated from the livers of young and old rats. These cells, known as hepatocytes, are convenient to isolate from animals, and use of the liver provides several advantages over other organs. First, the liver has high levels of vitamin C, underscoring its importance in this organ. Second, liver cells have been shown to have both SVCT1 and SVCT2, suggesting a reliance on vitamin C transport. To determine if vitamin C transport changes in these cells, we incubated hepatocytes from young and old rats with vitamin C and measured the amount of intracellular vitamin C over time. Both cells from young and old animals took up vitamin C rapidly from the medium, effectively doubling their vitamin C levels. However, cells isolated from old rats showed a 68% decline in both the rate of vitamin C uptake and levels of vitamin C after uptake compared to the cells from young rats. To confirm that this alteration is due to the action of the sodium-dependent transporter, we incubated the cells in sodium-free media before adding vitamin C. Under this condition we greatly reduced the rate of vitamin C uptake in cells from both young and old animals. This suggests that vitamin C uptake by these cells, as it is thought to occur in most cells, is due to the action of SVCT. Therefore, the decline in vitamin C transport seen in old rats may be due to a decline in SVCT.
To further characterize the age-related decline in vitamin C transport, we incubated the cells from young and old animals with increasing amounts of vitamin C. Although our previous experiments focused on normal plasma levels of vitamin C, we were interested to see if increased levels of vitamin C might be achieved, if only transiently, from supplementation. Our results showed that both cells from young and old rats took up vitamin C in a dose-dependent manner. However, the cells from old rats didn't achieve the levels of vitamin C observed in cells from young rats at any concentration of vitamin C in the culture medium. Furthermore, we found that cells from young rats have a greater capacity to transport vitamin C, as we expected. However, we did not expect to find that the cells from old rats have an increased affinity for vitamin C! This suggests that vitamin C transport is changing to a low capacity, yet higher affinity process with age. This can be explained by looking at the vitamin C transport protein isoforms individually. SVCT1 is a protein found in the liver, kidney, and intestine. It has been characterized as a high capacity transport protein, but its affinity for vitamin C transport is relatively low. On the other hand, SVCT2, also found in the liver and kidney, has been detected in tissues where vitamin C levels are high, such as the brain and eye. SVCT2 has been characterized as a high-affinity transport protein, but with much lower capacity than SVCT1. In light of the fact that both SVCT1 and SVCT2 are present in the liver, the observed changes in vitamin C uptake could be due to the preferential loss of one of these transport proteins.
Unfortunately, we are unable to detect levels of the SVCT proteins directly to determine if these transporters do change with age. Therefore, we used a method to determine the amount of messenger RNA (mRNA), which instructs the cell to produce the SVCT protein. While this doesn't determine the exact amounts of the protein, it is usually a good indicator of protein levels. Using liver tissue samples from young and old rats, we quantified the amount of mRNA for SVCT1 and SVCT2. As we expected, levels of SVCT1 mRNA decline nearly 45% with age. In contrast, levels of SVCT2 mRNA did not decline much at all. Together, these data suggest that there is a preferential loss of SVCT1 in liver from older animals and that losses in SVCT1 seem to be responsible for the decline in vitamin C uptake in the aging cell.
Now that we have established, to some extent, what causes a change in vitamin C transport in the aging cell, we have to understand why this change occurs before it can be remedied. Changes in SVCT mRNA levels suggest that the cell is not compensating for this loss of vitamin C transport properly, and there may be further inappropriate signals in the cells from old rats that result in the production of less protein. In order to understand the role that cell signaling has on SVCT function, we are trying to sequence the SVCT genes. Using this information we hope to understand more about the synthesis of SVCT proteins and, eventually, how we can reverse the age-related changes in vitamin C uptake. Since SVCT is present in both rats and humans, this research may be of great benefit to the elderly. We are excited about the many possibilities, including an observation that R-alpha-lipoic acid completely reverses the decline in vitamin C status in cells in old rats. Could this be due to changes in vitamin C transport? What is clear from our research and the research published by others is that vitamin C levels decline with age, just as the body appears to need it the most. Although good dietary practices may maintain adequate levels of vitamin C in young adults, we believe that a larger intake of vitamin C may be required in the elderly to lead to optimal health.
For more information on vitamin C, see the Linus Pauling Institute's Micronutrient Information Center.
Last updated November, 2002
Micronutrient Research for Optimum Health
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